Ferreira and Russ: Age-validation and growth rate of Plectropomus leopardus 



49 



fence nets. Three of these fishes were injected with 

 tetracycline at the time of capture, and all five fish 

 were kept in captivity for periods of 3 to 17 months. 

 The otoliths of the fishes treated with tetracycline 

 were removed, sectioned, and observed under fluo- 

 rescent light. To determine time of formation of the 

 translucent and opaque zones, the distances be- 

 tween events for which time of occurrence was 

 known (i.e., between two tetracycline bands or be- 

 tween a tetracycline band and the margin of the 

 otolith) were measured on otolith sections and plot- 

 ted against the corresponding time interval. The 

 relative positions of the translucent and opaque 

 zones to these marks were then measured and plot- 

 ted on the same scale. While this method does not 

 provide real distances, it standardizes the measure- 

 ments allowing for comparison between fish of dif- 

 ferent ages. 



The relation between otolith weight, fish size 

 (length and weight), and age was analyzed. Otolith 

 weight was plotted against FL for each age class 

 separately. A multiple linear regression model was 

 fitted in a step-wise manner to predict age from 

 otolith weight and fish size and to predict otolith 

 weight from age and fish size. The inclusion level 

 for the independent variables was set at P=0.10. The 

 assumptions of normality and homoscedasticity 

 were tested by plotting the residuals from the re- 

 gression models. 



The growth models were fitted to the data and 

 their coefficients and standard errors estimated by 

 means of standard non-linear optimization methods 

 (Wilkinson, 1989). As the plot of the length-at-age 

 data indicated, some form of asymptotic growth, 

 Schnute's (1981) reformulation of the von Bert- 

 alanffy growth equation for length in which a*0 was 

 fitted to the data: 



,-aU-t\) 



L,=y\ h +(y2 b -yl b ) 



where L f is length at age; tl and t2 are ages fixed 

 as 1 and 14 respectively ; yl and y2 are estimated 

 sizes at these ages; and a and b are the parameters 

 which indicate if the appropriate growth curve lies 

 closer to a three or two parameter sub-model. By 

 limiting parameter values, the data were used di- 

 rectly in selecting the appropriate sub-model, 

 namely the generalized von Bertalanffy, Richards, 

 Gompertz, Logistic, or Linear growth models. Sub- 

 sequently, the original von Bertalanffy (1938) 



-KU -to) 



To evaluate the effects of gear selectivity (and 

 consequently varying size and age composition) on 

 the estimates of growth parameters, the von 

 Bertalanffy growth equation was fitted first to data 

 collected by line and spear fishing only and then to 

 the same data combined with the fence-net sample 

 composed of younger fish. 



Results 



Otolith reading 



In the coral trout, the sagittae presented a pattern 

 of alternating translucent zones and wide opaque 

 zones (annuli) with no sharp contrast between zones 

 (Fig. 1). The first two annuli were notably wider and 

 less well defined than the subsequent ones in sec- 

 tioned otoliths. Whole sagittae were used to confirm 

 the presence of these first annuli. 



In whole otoliths, annuli were clearly distinguish- 

 able and easy to count along the dorsal side of the 

 otolith, where up to 12 rings were counted in some 

 otoliths. However, readings from whole otoliths 

 tended to be lower than readings from sectioned 

 otoliths when more than six rings were present, and 

 this tendency increased with the mean number of 

 rings, particularly after ten rings. (Fig. 2 ). Tetra- 

 cycline-labelled otoliths validated the periodicity of 

 annuli in sectioned otoliths, indicating that whole 

 otolith readings tend to underestimate age of > 10- 

 year-old fishes. A comparison between results of 



)was 



growth equation for length L ( = L^d-e 

 fitted to the data. V is length at age; L x is the as- 

 ymptotic length, K is the growth coefficient, t is age, 

 and t o is the hypothetical age at which length is zero. 



